1. Field Of The Invention
The present invention relates to an improved composite center electrode for a spark plug and to a method for producing same, wherein the electrode includes a sparking tip composed of a precious metal.
Because spark plugs are typically utilized in highly corrosive environments, it is desirable to form the center electrodes thereof of a material having a high degree of resistance to corrosion, such as nickel, nickel alloy, or the like. It also is desirable to have a high thermal conductivity material in the center electrode to increase the operating range for the spark plug. Accordingly, it is known to form the spark plug center electrode with an inner core of a high thermal conductivity material, such as copper, surrounded by an outer protective sheath of corrosion-resistant material. It is also known to incorporate precious or semi-precious metals in the tip of the spark plug center electrode for prolonged life. Such metals are very resistant to corrosion at the high temperatures encountered in the combustion chamber of an engine and to erosion from spark discharge. However, such precious and semi-precious metals are very expensive, and their inclusion typically requires several additional steps in the process of forming the spark plug electrode.
2. Description Of The Prior Art
U.S. Pat. No. 3,144,576 to Hagmaier et al. discloses a method for producing a composite center electrode for a spark plug by initially superposing a right circular cylindrical plate of a metal having good thermal conductivity, such as copper, upon a right circular cylindrical plate of corrosion-resistant metal, such as nickel. The superimposed plates are placed within a die and extruded therethrough to form a rod consisting of a core having good thermal conductivity within a corrosion-resistant shell.
U.S. Pat. No. 3,548,472 to Urushiwara et al. discloses another method for producing a composite center electrode for a spark plug. Initially, a right circular cylindrical billet of a corrosion-resistant metal is subjected to successive extrusions and drawings to form an elongated cup having a cavity extending therein. A right circular cylindrical billet of a metal having a relatively higher thermal conductivity is then extruded to form a core having a headed portion with a diameter equal to that of the elongated cup and a protruding portion of reduced diameter slightly less than that of the cavity. The protruding portion of the core is inserted into the cavity of the cup and pressed therein to form an integral rod, which is subjected to heat diffusion. A composite center electrode is then formed by cold working the rod to form a head thereon. Both the elongated cup and the core are worked separately to dimensions substantially the same as their final dimensions before the heat diffusion step.
U.S. Pat. No. 3,857,147 to Yamaguchi et al. discloses a method of producing a composite center electrode for a spark plug. A right circular cylindrical billet of a corrosion-resistant metal is subjected to a single extrusion step to form a cup having a cavity extending therein. A right circular cylindrical billet of a metal having good thermal conductivity is then extruded to form a cap having a headed portion with a diameter equal to that of the cup and a protruding portion of reduced diameter slightly less than that of the cavity. The protruding portion of the cap is inserted into the cavity of the cup, and both are extruded through a die to form a rod having a core of uniform diameter within a shell having walls of uniform thickness. A composite center electrode is then formed by cold working the rod to form a head thereon.
U.S. Pat. Nos. 3,868,530 and 3,967,149, both to Eaton et al., disclose a spark plug having a center electrode formed of a corrosion-resistant metal. An axial recess is formed in a flared end of the electrode, and an insert of precious metal is inserted therein to fill the recess. A swaging operation is then performed to return the flared end of the center electrode to a cylindrical configuration and thereby retain the precious metal inserts therein.
Other related processes and constructions are described and illustrated in U.S. Pat. Nos. 1,521,732 to Thomas, 1,789,078 to McElroy, 2,849,788 to Creek, 2,945,293 to Last, 2,954,495 to Zeller, 2,955,222 to Beesch, 3,040,417 to Newton, 3,548,239 to Eaton, 3,643,322 to Gerstle, 3,803,892 to Yamaguchi et al., and 3,818,555 to Yamaguchi et al., and also in French Pat. No. 819,156 and German Pat. No. 1,052,781.